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<TITLE>LambdaMOO Programmer's Manual - Passing</TITLE>
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<H3><A NAME="SEC40" HREF="ProgrammersManual_toc.html#TOC40">Object-Oriented Programming</A></H3>
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One of the most important facilities in an object-oriented programming language
is ability for a child object to make use of a parent's implementation of some
operation, even when the child provides its own definition for that operation.
The <CODE>pass()</CODE> function provides this facility in MOO.
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<DT><U>Function:</U> value <B>pass</B> <I>(<VAR>arg</VAR>, ...)</I>
<DD><A NAME="IDX1"></A>
Often, it is useful for a child object to define a verb that <EM>augments</EM>
the behavior of a verb on its parent object. For example, in the LambdaCore
database, the root object (which is an ancestor of every other object) defines
a verb called <SAMP>`description'</SAMP> that simply returns the value of
<CODE>this.description</CODE>; this verb is used by the implementation of the
<CODE>look</CODE> command. In many cases, a programmer would like the description of
some object to include some non-constant part; for example, a sentence about
whether or not the object was `awake' or `sleeping'. This sentence should be
added onto the end of the normal description. The programmer would like to
have a means of calling the normal <CODE>description</CODE> verb and then appending
the sentence onto the end of that description. The function <SAMP>`pass()'</SAMP> is
for exactly such situations.
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<CODE>pass</CODE> calls the verb with the same name as the current verb but as
defined on the parent of the object that defines the current verb. The
arguments given to <CODE>pass</CODE> are the ones given to the called verb and the
returned value of the called verb is returned from the call to <CODE>pass</CODE>.
The initial value of <CODE>this</CODE> in the called verb is the same as in the
calling verb.
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Thus, in the example above, the child-object's <CODE>description</CODE> verb might
have the following implementation:
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return pass() + " It is " + (this.awake ? "awake." | "sleeping.");
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That is, it calls its parent's <CODE>description</CODE> verb and then appends to the
result a sentence whose content is computed based on the value of a property on
the object.
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In almost all cases, you will want to call <SAMP>`pass()'</SAMP> with the same
arguments as were given to the current verb. This is easy to write in MOO;
just call <CODE>pass(@args)</CODE>.
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